How to create triangle/jagged edges on a cylinder (e.g. for a lamp shade)
This is pretty straight forward. The advantage of this technique is it keeps the crease depth equal from top to bottom, just like a real world lamp shade.




Create a cylinder. Choose No Caps, and set the subdivision to 64.
Scale the cylinder on the Z axis (S+Z) to get the correct height.
Tab into Edit mode, switch selection mode to Edges
Select one vertical edge, then in the Select menu choose Select Similar>Length to select all the vertical edges
Select Checker Deselect in the Select menu to deselect every other edge
Click S+SHIFT+Z to scale the selected edges in towards the centre of the cylinder
Tab into Object Mode and add the Simple Deform modifier
Adjust the Deform Factor to a negative value to get a narrow top and wide bottom for the lampshade.